Mossbauer spectrometer

ABSTRACT

A Mossbauer spectrometer includes a source shielded by a layer of depleted uranium covered with a layer of graded shielding material. A detector is placed so that it is shielded from the gamma radiation from the source. The sample being examined is inclined at an angle to the beam of gamma rays from the source so that the resonant and nonresonant gamma-ray energy is directed to the detector.

United States Patent Richard Neil 0rd Richland, Wash. 382,380 Oct. 15,1970 ou. 12, 1971 The United States of America as represented by theUnited States Atomic Energy Commission Inventor Appl. No. Filed PatentedAssignee MGSSBAUER SPECTROMETER 5 Claims, 1 Drawing Fig.

US. Cl 250/83.3 R, 250/84, 250/108 R int. Cl G0lt 1/16 Field of Search250/83.3 R,

. Primary Examiner-Archie R. Borchelt [56] References Cited UNITEDSTATES PATENTS 3,454,766 7/1969 Roughton .1. 250/84 X 3,506,829 4/1970Hannan 250/83.3 R

Attorney-Roland Anderson ABSTRACT: A Mb'ssbau er spectrometer includes,a source shielded by a layer of depleted uranium covered with a layerof graded shielding material. A detector is placed so thatit is shieldedfrom the gamma radiation from the source. The sample being examined isinclined at an angle to the beam of gamma rays from the source so thatthe resonant and nonresonant gamma-ray energy is directed to thedetector.

1 MossBAUER SPECTROMETER CONTRACTUAL ORIGIN OF THE INVENTION Theinvention described herein was made in the course of, or under, acontract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION In M'ossbauer spectroscopy a radioactivesource consisting of excited nuclei decays to the ground state and in sodoing emits highly monochromatic gamma radiation. The emitted gamma raysare resonantly absorbed in nuclei within a sample where the nuclei arecharacteristic of the ground state nuclei of the radioactive source. Theresonance line is so narrow that its energy can be shifted significantlyby means of the Doppler effect, that is by moving the source (orabsorber) at velocities of the order of a few mm./sec.

The conventional technique used in Mo'ssbauer spectroscopy is to locatea thin sample between the radioactive source and an appropriatedetector. This technique has the disadvantage that the specimen beingexamined is radically altered to prepare it for test. The technique isthus not suitable for nondestructive test purposes.

Mossbauer spectrometers have been made which use a reflection orbackscatter technique. However, the geometry used has been such that thepercentage resonance elfect magnitude (R) is low (on the order of 30%).[R =I() !(w)/I( 100] where 1(0) is the resonant count and 1( is thenonresonant count. Changes in shielding by the use of heavy metals haveonly given a slight improvement. The counting time for prior artMo'ssbauer spectrometers is also very long (many hours) and energylevels of the gamma rays are not easily detected. Increasing theradioactivity of the source to decrease the counting time has an adverseeffect on the percentage resonance effect magnitude, thereby decreasingresolution.

It is therefore an object of this invention to provide an improvedMossbauer spectrometer.

Another object of this invention is to provide a Mossbauer spectrometersuitable for nondestructive testing.

Another object of this invention is to provide a Mb'ssbauer spectrometerwhich has short counting times.

Another object of this invention is to provide a Mb'ssbauer spectrometerwhich has improved resolution.

SUMMARY OF THE INVENTION In practicing this invention a Mossbauerspectrometer is provided which includes a high-energy gamma source in ashielded enclosure. The enclosure has a conical collimator as an exitport to provide a collimated beam of gamma rays. The beam of gamma raysis directed against the sample under examination. The surface of thesample is inclined at an angle to the beam of gamma rays so that theresonant and nonresonant gamma-ray energy is directed to a detectorpositioned out of the path of beam of gamma rays. The detector is placedso that it is completely shielded from the gamma radiation from thesource.

The shielding for the enclosure includes a layer of depleted uraniumcovered with a layer of graded shielding material. The source is movedat a constant acceleration relative to the sample to sweep the samplewith a narrow range of gamma-ray energies.

BRIEF DESCRIPTION OF THE DRAWING The invention is shown in the singledrawing, a sectional view of the M'ossbauer spectrometer.

DETAILED DEsCR'IFTKiN or TfiE INVENTION Referrin mire dismissal;SEWHTgZrFEa-ray shield consisting of a layer of copper l6 andpolyethylene 17. An exit port 19 permits gamma radiation to leave theshielded enclosure 11. The exit port 19 is conical in shape to provide acollimated beam of electrons.

A sample to be examined 20 is positioned opposite exit port 19 and at anangle 0 to the gamma radiation from source 10. Angle 0 may convenientlybe 45 but it is not restricted to this angle.

A detector 22 is positioned to receive backscatter Mo'ssbauer gammaradiation from the sample 22 and develops an output signal in responseto the gamma radiation detected. Detector 22 is also positioned withshielded enclosure 11 between source 10 and detector 22 so that detector22 sees only the backscatter gamma radiation from sample 20 and does notsee the gamma radiation from source 10. The angle 1 is chosen to achievethe best possible isolation of detector 22 from source 10 gammaradiation.

The output signal from detector 22 is amplified in preamplifier 24,amplifier 26 and single-channel analyzer 27 and recorder in multichannelanalyzer 25. Source drive unit 13 acts to drive source 10 toward andaway from sample 20 with a constant acceleration so that the energy ofthe gamma radiation from source 10 sweeps through a desired energyrange.

' The multichannel analyzer 25 is coupled to drive unit 13 so that theproper channel is selected according to the instantaneous velocity ofthe source 10.

In an example of a Mb'ssbauer effect spectrometer using the features ofthe invention, the source 10 was mCi of cobalt-57 deposited on apalladium matrix. The resonance effect magnitude was greater than 200percent as compared with resonance effect magnitude of the order of30percent for prior art Mo'ssbauer spectrometers.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A Mo'ssbauer backscatter spectrometer for nondestructive analysis ofa material sample, including in combination, a high-energy gamma source,a radiation shield positioned around said gamma source, said radiationshield including a layer of depleted uranium having an exit porttherein, means for positioning said sample opposite said exit port inthe path of said gamma radiation from said source, said sample beingpositioned at an angle to said path of said gamma radiation wherebybackscattered gamma therefrom is directed away from said path of saidgamma radiation, detector means positioned to receive said backscattergamma from said sample, said detector means further being positioned sothat said shielding is between said source and said detector means.

2. The M'ossbauer backscatter spectrometer of claim 1 wherein, saidradiation shield includes a graded shield surrounding said depleteduranium, said graded shield including a layer of polyethylene and alayer of copper positioned between said polyethylene layer and saiddepleted uranium.

3. The Mb'ssbauer backscatter spectrometer of claim 2 further including,source driving means positioned within said radiation shield, saidsource being mounted on said source driving means, said source drivingmeans acting to move said source alternately toward and away from saidexit port at a constant acceleration whereby the energy of said gammaradiation is swept across a desired range.

4. The Mdssbauer backscatter spectrometer of claim 3 wherein, said-exitport is conical in shape.

5. The Mo'ssbauer backscatter spectrometer of claim 4 wherein, saidsample is inclined at an angle of 45 to said path of said gammaradiation.

1. A Mossbauer backscatter spectrometer for nondestructive analysis of amaterial sample, including in combination, a highenergy gamma source, aradiation shield positioned around said gamma source, said radiationshield including a layer of depleted uranium having an exit porttherein, means for positioning said sample opposite said exit port inthe path of said gamma radiation from said source, said sample beingpositioned at an angle to said path of said gamma radiation wherebybackscattered gamma therefrom is directed away from said path of saidgamma radiation, detector means positioned to receive said backscattergamma from said sample, said detector means further being positioned sothat said shielding is between said source and said detector means. 2.The Mossbauer backscatter spectrometer of claim 1 wherein, saidradiation shield includes a graded shield surrounding said depleteduranium, said graded shield including a layer of polyethylene and alayer of copper positioned between said polyethylene layer and saiddepleted uranium.
 3. The Mossbauer backscatter spectrometer of claim 2further including, source driving means positioned within said radiationshield, said source being mounted on said source driving means, saidsource driving means acting to move said source alternately toward andaway from said exit port at a constant acceleration whereby the energyof said gamma radiation is swept across a desired range.
 4. TheMossbauer backscatter spectrometer of claim 3 wherein, said exit port isconicAl in shape.
 5. The Mossbauer backscatter spectrometer of claim 4wherein, said sample is inclined at an angle of 45* to said path of saidgamma radiation.